1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system or an electrostatic recording system, for example, an image forming apparatus such as a copying machine, a printer or a facsimile.
2. Related Background Art
Up to now, a developing device equipped in an image forming apparatus of the electrophotographic system or the electrostatic recording system uses a two-component developer essentially including toner particles and carrier particles. In particular, in a color image forming apparatus that forms a full color or a multicolor image through the electrophotographic system, most of the developing devices use two-component developers from the viewpoint of hue, tone, or the like of the image.
As well known, the toner density of the two-component developer, that is, a rate of the toner weight to the total weight of the carrier particles and the toner particles becomes a very important element in stabilization of the image quality. The toner particles of the developer is consumed at the time of development, and thereafter the toner density of the developer is reduced. For that reason, it is important that an automatic toner replenishing controller (ATR) is employed to accurately detect the toner density of the developer in good time, to replenish the toner in accordance with a change in the toner density, always to constantly control the toner density and keep the image quality.
As described above, in order to correct the change in the toner density within the developing device due to development, that is, in order to control the toner amount which is replenished to the developing device, developer density detectors of various types which are disposed within the developing container have been put in practical use.
For example, there has been employed an optical developer density controller, a developer density controller of an inductance detecting type, or the like, which is disposed in the vicinity of a developer bearing member (hereinafter referred to as “developing sleeve” since, in general, the developing sleeve is frequently used), or a developer carrying path of the developing container. The optical developer density controller grasps the toner density and controls the toner amount which is replenished to the developing device by utilizing a phenomenon that a reflection factor of a light irradiated onto a developer carried on the developing sleeve or a developer within the developing container is different depending on the toner density. The developer density controller of the inductance detecting type grasps the toner density within the developing container in accordance with a detection signal from an inductance head that detects an apparent magnetic permeability due to the mixture ratio of the magnetic carrier and the nonmagnetic toner in the developer and converts the detection signal from the inductance head into an electric signal, and replenishes the toner on the basis of a comparison of the toner density with a reference value.
Also, there is a system in which an image density of a patch formed on an image bearing member (hereinafter referred to as “photosensitive drum” since in general, the photosensitive drum is frequently used) is read by a light source disposed on a position that faces the surface of the image bearing member and a sensor that receives its reflected light, and then converted into a digital signal by an analog-to-digital converter, and thereafter the digital signal is transmitted to a CPU and then compared with an initial set value in the CPU. In the system, if the image density is higher than the initial set value, the toner replenishment stops until the image density returns to the initial set value whereas if the image density is lower than the initial set value, the toner is forcedly replenished until the image density is returned to the initial set value, and the toner density is indirectly maintained to a desired value.
Further, there is a developer density controller called “a video count system” in which a consumed toner amount is estimated on the basis of a video count of an image density signal corresponding to the image information of an original read by such as the CCD, and the toner corresponding to the estimated consumed toner amount is replenished.
However, in the system the toner density is detected from the reflection factor of the light irradiated onto the developer carried on the developing sleeve or the developer within the developing container, if the sensor is stained by toner scattering or the like, there is a case in which the toner density cannot be accurately grasped and detected.
Also, in the system that indirectly controls the toner density from the patch image density, with the downsizing of the image forming apparatus, disenables a space where a patch image is formed or a space where a detecting means is located to be ensured.
Further, the toner replenishment due to the video count system is so controlled as to be set to an appropriate developer density more rapidly than if more toner is consumed due to a high-density image since the toner replenishment amount is calculated every time the image forming operation is conducted.
However, in the case where there is even a slight difference between the consumed toner amount calculated on the basis of the video count and the toner replenishment amount due to a precision of the toner hopper that conducts the toner replenishment or the like, if images are formed (developed) on a large amount of transfer materials (paper or the like), because the developer density is gradually shifted from an initial appropriate developer density, it may be difficult to control the developer density with only the video count system.
On the other hand, the above developer density controller of the inductance detecting system (hereinafter referred to as “inductance detecting system ATR”) does not suffer from the above problem, and controls the toner amount which is replenished to the developing device on the basis of the following control. That is, for example, if it is detected that the apparent magnetic permeability of the developer is large, a rate at which the carrier particles in the developer is occupied in a constant volume becomes large, which means that the toner density becomes low. Therefore, the toner replenishment starts. Conversely, if the apparent magnetic permeability becomes small, the rate at which the carrier particles in the developer is occupied in the constant volume becomes small, which means that the toner density becomes high. Therefore, the toner replenishment stops.
However, in the above-mentioned inductance detecting system ATR, there is a case in which the output from the inductance head in correspondence with the apparent magnetic permeability is discontinuously changed due to a change in a bulk density of the developer due to the leaving of the developer or an environmental variation between a time immediately before the operation of the image forming apparatus stops (for example, immediately before the main switch of the image forming apparatus is switched off) and a time immediately after the image forming apparatus restarts (for example, immediately after the main switch of the image forming apparatus is switched on).
In other words, that the bulk density of the developer is changed within the developing container even if the toner amount in the developing container does not substantially change between a time immediately before the operation of the image forming apparatus stops (for example, immediately before the main switch of the image forming apparatus turns off) and a time immediately after the operation restarts (for example, immediately after the main switch of the image forming apparatus turns on), means that the amount of developer (carrier particles)within the constant volume in the vicinity of the sensor changes in the inductance detecting system ATR. As a result, there is a case in which the toner is replenished in response to an output signal from the head indicating that the toner has been decreased regardless of the toner amount being not substantially changed.
In this case, there may occur a problem that the image density becomes high due to the excessive toner replenishment, a problem that the amount of developer increases with an increase of the toner amount to the degree that the developing container overflows with the developer, or a problem that the toner is scattered due to a deterioration of the charge amount of the toner with an increase in the ratio of toner in the developer.
Also, the above-described problems are particularly remarkable in the case where a stop period of time after the stoppage of the image forming apparatus and before the restart of the image forming apparatus is long, or in the case where the environmental variation is large during that stop period of time.